Sharp‐based marine sandstone bodies in the Mulichinco Formation (Lower Cretaceous), Neuquén Basin, Argentina: remnants of transgressive offshore sand ridges

Abstract

The interpretation of sharp‐based shallow‐marine sandstone bodies encased in offshore mudstones, particularly transgressive units, has been a subject of recent debate. This contribution provides a multiple‐dataset approach and new identification criteria which could help in the recognition of transgressive offshore sandstone bodies worldwide. This study integrates sedimentology, ichnology, taphonomy and palaeoecology of Mulichinco Formation strata in the central Neuquén Basin (Argentina) in order to describe and interpret sharp‐based sandstone bodies developed in ramp‐type marine settings. These bodies are sandwiched between finer‐grained siliciclastics beneath and thin carbonates above. The underlying sediments comprise progradational successions from offshore mudstones to offshore transition muddy sandstones, grading occasionally into lower shoreface sandstones. The surfaces capping the regressive siliciclastics are flat and regionally extensive, and are demarcated by skeletal concentrations and a Glossifungites suite; they are also marked by sandstone rip‐up clasts, with encrustations and borings on all sides. These surfaces are interpreted as composite discontinuities, cut during a relative sea‐level fall and remodelled during the initial transgression. The overlying transgressive sandstone bodies are 3 to 7 m thick, >4 km long and about three times longer than wide; they are composed of fine‐grained sandstones with little lateral change in grain size. Cross‐stratification and/or cross‐lamination are common, typically with smaller‐scale structures and finer grain size towards the top. Large‐scale, low‐angle (5° to 8°) inclined stratification is also common, dipping at ca 30° with respect to body elongation and dominant currents. These sandstone bodies are interpreted as offshore sand ridges, probably developed under the influence of tidal currents. Intense burrowing is typical at the top of each unit, suggesting an abandonment stage. Final deactivation favoured colonization by epibenthic‐dominated communities and the formation of skeletal‐rich limestones during the latest transgressive conditions. As partial reworking of pre‐existing ridges occurred during this stage, the Mulichinco sandstone bodies are considered the remnants of transgressive offshore sand units.

Middle Turonian trace fossils from the Bystrzyca and Długopole sandstones

“…Pem ber ton et al (2001), who re ferred on a sim i lar as sem blage of trace fos sils, placed the suc ces sion con tain ing Thalassinoides, Palaeophycus and Ophiomorpha in the up per off shore, whereas de pos its rich in Ophiomorpha, Macaronichnus in the mid dle-lower shoreface. In com par i son to other nearshore trace fos sils as sem blages (lower shoreface-up per off shore set tings) in clud ing Macaronichnus, Ophiomorpha, Palaeophycus, Thalassinoides, the ma jor ity of them were re corded from the storm-orig i nated de pos its (Frey and Pem ber ton, 1991;MacEachern et al, 1999;Bann and Fieding, 2004;Bann et al, 2004;Gani et al, 2008;Rygel et al, 2008;Schwarz, 2012) or bar set tings (Curran, 1985;Pol lard et al, 1993;Olariu et al, 2012;Sullivan and Sullivan, 2012).…”

Palaeoecological analysis of a sclerobiont fauna on a single basibiont across the Valanginian of the Neuquén Basin, west-central Argentina

“…Arroyo Truquico and Cerro Caicayén are entirely within the LST. Materials from the TST are scarce ( N = 62 valves, or 31 articulated specimens), since this part of the sequence is the thinnest (as TST deposits often are; Patzkowsky & Holland ; see Schwarz , Fig. ), and they come from Pichaihue, Bajada Vieja and El Salado sections.…”

“…Chart displaying second‐order systems tracts in relation to ammonite zonation (see Aguirre‐Urreta et al . , , ; Schwarz ; Schwarz & Buatois ). Vertical bars correspond to the range covered by each section.…”

Architecture and recognition criteria of ancient shelf ridges; an example from Campanian Almond Formation in Hanna Basin, USA

“…). This is a characteristic feature in tidal‐shelf ridges recognized in modern environments (Houbolt, ; Reynaud et al ., ; Trentesaux et al ., ; Dalrymple, ), as well as in other ancient tidal ridges and bars (Dalrymple & Rhodes, ; Olariu et al ., ; Schwarz, ; Rossi & Steel, ).…”

“…Shelf sand ridges (storm or tide‐dominated) are thought to be formed during transgressions, as a result of storm and tidal currents reworking lowstand (coarse‐grained) deposits, and therefore they overlie a transgressive surface of erosion (Posamentier, ; Snedden et al ., ; Schwarz, ). Very few examples of transgressive shelf ridges have been identified in the geological record (Michaud, ; Olariu et al ., ; Schwarz, ; Messina et al ., ) when compared to their abundance in modern shelf systems (Houbolt, ; Swift & Field, ; Thomas & Anderson, ; Berné et al ., , ; Reynaud et al ., ; Snedden & Dalrymple, ; Trentesaux et al ., ; van de Meene & van Rijn, ; Snedden et al ., ).…”